Official White House Photo by Pete Souza(WASHINGTON) -- The leaders of the U.S. and Russia traded warnings Thursday as the crisis in the Ukraine becomes more serious by the day.

In Tokyo, President Obama said that more U.S. sanctions on Russia are “teed up and ready to go,” signaling that if Moscow does not reverse course in eastern Ukraine, it will face additional consequences “in days not weeks.”

The U.S. and its allies have accused Russia of fomenting unrest following the ouster of Ukraine's pro-Moscow president in late February that led to an incursion into Crimea and the rise of separatists in eastern Ukraine who are demanding their region become part of the Russian Federation.

But even with further sanctions, Obama acknowledged it is “entirely possible” that new economic penalties will not work to change Russian President Vladimir Putin’s calculus on Ukraine.

“So far the evidence doesn’t make me hopeful,” Obama said. “Assuming they don’t follow through, then we’ll follow through on what we said…which is tighter consequences on the Russians.”

Meanwhile, Putin offered a vague but ominous warning to Ukraine’s government, saying there may be unspecified “consequences” for its use of force against pro-Russian separatists in the east.

“If the Kiev regime has started to use the army against the population inside the country, it, beyond any doubt, is a very serious crime,” Putin said, according to Russia’s Interfax news agency.

“It will certainly have consequences for the people who make such decisions,” he added. “This also concerns our intergovernmental relations.”

His comments renewed fears that Russian troops, who are amassed near the border of eastern Ukraine, might launch an invasion under the pretext of protecting ethnic Russians there.

Later on Thursday, Secretary of State John Kerry joined in the fray by accusing Russia of doing nothing to honor the Geneva Agreement that called for separatists to stand down in eastern Ukraine.

In a message directly to Putin, Kerry added the Russian president would see additional sanctions on individuals and businesses in his country but did not make any mention of when they would be imposed.